It is known that fear, dread, panic, and excruciating pain may accompany unintended awareness during anesthesia. During this anesthesia state, patients may recall events or conversations that occurred in the operation room. These incidents are not benign. For example, some incidents have resulted in posttraumatic stress disorder. Prevention of unintended awareness during anesthesia may be accomplished by having an alert anesthesia clinician monitor an anesthetized patient's vital signs. However, vital signs may not provide a sufficient warning. For example, vital signs may not provide a sufficient warning in elderly anesthetized patients with comorbid conditions such as hypertension or tachycardia.
There has been interest in developing state of anesthesia devices that can continuously and reliably monitor the anesthesia state during a surgical procedure. Such devices have largely been based on the measurement of electrophysiological signals such as electrocardiographic (ECG) signals, electroencephalographic (EEG) signals, auditory and somatosensory evoked potentials, and craniofacial electromographic (EMG) signals. Unfortunately, electrophysiological parameters provide limited accuracy as those parameters involve measuring electric currents and cannot measure biological parameters such as hemodynamic response.